Reaction studies at high pressure. Part I. Activation volumes of some[2 + 2] and dipolar cycloadditions

Activation volumes have been determined over the pressure range 0–2 kbar for several [2 + 2] cycloadditions both with concerted and non-concerted mechanisms and also for a 1,3-dipolar cycloaddition. Values of ΔV are found to be large and negative there being little distinction between mechanistic types for the [2 + 2] reactions. The 1,3-dipolar cycloaddition and the dimerisation of dimethylketen show values comparable with those of Diels–Alder reactions. The implications of these findings are discussed.